1. Field of the Invention
The present invention relates to a positive resist composition used in a manufacturing process of semiconductors, such as IC, manufacture of circuit substrates for liquid crystals, thermal heads and the like, and lithographic process of other photo-fabrication, and also relates to a pattern forming method using the same. In particular, the invention relates to a positive resist composition for immersion exposure suitable for exposure with an immersion projection exposure apparatus using far ultraviolet rays of wavelengths of 300 nm or less as the light source, and a pattern-forming method using the same.
2. Description of the Related Art
With the progress of fining of semiconductor elements, shortening of the wavelengths of exposure light source and increasing of the numerical aperture of the projection lens have advanced, and now exposure apparatus of NA 0.84 using an ArF excimer laser having wavelength of 193 nm as the light source have been developed. As generally known, these can be expressed by the following expressions:(Resolution)=k1·(λ/NA)(Depth of focus)=±k2·λ/NA2 wherein λ is the wavelength of an exposure light source, NA is the numerical aperture of the projection lens, k1 and k2 are the coefficients concerning the process.
For further higher resolution by the shortening of wavelengths, an exposure apparatus with an F2 excimer laser having wavelength of 157 nm as the light source has been studied, however, the materials of lens for use in the exposure apparatus and the materials of resist are extremely restricted for shortening of wavelengths, so that the realization of the reasonable manufacturing costs of the apparatus and materials and quality stabilization are very difficult, as a result, there are possibilities of missing an exposure apparatus and a resist having sufficient performances and stabilities within a required period of time.
As a technique for increasing resolution in optical microscope, a so-called immersion method of filling between a projection lens and a sample with a liquid of high refractive index (hereinafter also referred to as “immersion liquid”) has been conventionally known.
As “the effect of immersion”, the above resolution and depth of focus can be expressed by the following expressions in the case of immersion, taking λ0 as the wavelength of the exposure light in the air, n as the refractive index of immersion liquid to the air, and NA0=sinθ with θ as convergence half angle of the ray of light:(Resolution)=k1·(λ0/n)/NA)0 (Depth of focus)=±k2·(λ0/n)/NA02 
That is, the effect of immersion is equivalent to the case of using exposure wavelength of the wavelength of 1/n. In other words, in the case of the projection optical system of the same NA, the depth of focus can be made n magnifications by immersion. This is effective for every pattern form, and can be combined with super resolution techniques such as a phase shift method and a deformation lighting method.
The apparatus applying this effect to the transfer of micro-fine image pattern of semiconductor element are introduced by JP-A-57-153433 and JP-A-7-220990.
The latest technical advancement of immersion exposure is reported in SPIE Proc., 4688, 11 (2002), J. Vac. Sci. Tecnol. B, 17 (1999), SPIE Proc., 3999, 2 (2000), and WO 2004/077158. When an ArF excimer laser is used as the light source, it is thought that pure water (refractive index at 193 nm: 1.44) is most promising in the light of the safety in handling, and the transmittance and the refractive index at 193 nm. When an F2 excimer laser is used as the light source, a solution containing fluorine is discussed from the balance of the transmittance and the refractive index at 157 nm, but a sufficiently satisfactory solution from the viewpoint of environmental safety and refractive index has not been found yet. From the extent of the effect of immersion and the degree of completion of resist, it is thought that immersion exposure technique will be carried on an ArF exposure apparatus earliest.
From the advent of the resist for a KrF excimer laser (248 nm) on, an image-forming method that is called chemical amplification is used as the image-forming method of the resist for compensating for the reduction of sensitivity by light absorption. To explain the image-forming method of positive chemical amplification by example, this is an image-forming method of exposing a resist to decompose an acid generator in the exposed area to thereby generate an acid, utilizing the generated acid as the reactive catalyst to change an alkali-insoluble group to an alkali-soluble group by the bake after exposure (PEB: Post Exposure Bake), and removing the exposed area by alkali development.
The resist for an ArF excimer laser (wavelength: 193 nm) using the chemical amplification mechanism is now being a main current, but the improvement of line edge roughness is required when used in immersion exposure.
When a chemical amplification resist is applied to immersion exposure, it is appointed that since the resist layer inevitably touches an immersion liquid at the time of exposure, the resist layer decomposes and ingredients that adversely influence the immersion liquid ooze from the resist layer. WO 2004/068242 discloses that the resist performance changes by the immersion of a resist for ArF exposure in water before and after exposure and appoints this is a problem in immersion exposure.
Further, when exposure is performed with a scanning system immersion exposure in an immersion exposure process, the speed of exposure lowers if an immersion liquid does not move following the movement of a lens, so that there is the fear of influence on productivity. In the case where the immersion liquid is water, the resist film is preferably hydrophobic in view of good following ability of water.